Precipitation sensors have been developed to determine the presence of water in its vapor, liquid and solid forms, but usually the sensor is immersed in the material. Non-immersed sensing is a significant challenge. One example of a non-immersed sensor is the Bosch vehicle windshield rain sensor (Optical Sensor U.S. Pat. No. 6,376,824 by Michenfelder et al) used to operate windshield wipers. This sensor depends on the change in refraction of a reflected light beam against glass when water is on the outer glass surface. However, it has poor sensitivity for snow, unless the glass can be heated enough to melt the snow next to the glass. This would be difficult to facilitate without making the vehicle occupants too uncomfortable and initially, in cold environments, would not work at all until the heating reached an acceptable level for the sensor to be engaged.